1. Field of the Invention
The present invention relates to an electronic device, and more particularly to an electronic device of the type whose output terminal voltage is pulled up or down.
2. Related Background Art
An example of a conventional pull-up circuit is shown in FIGS. 4A and 4B. In FIG. 4A, a pull-up circuit with an input terminal Vin and an output terminal Vout is made of a p-channel MOS transistor 8 whose gate is grounded. As the input Vin changes from 0 to Vdd, current Id drained from the pull-up circuit changes from Idd to 0 as shown in FIG. 5A. In a pull-up circuit shown in FIG. 4B, current Id drained from the pull-up circuit or a resistor of high resistance value changes as shown in FIG. 5B. If such a pull-up circuit is applied to an input-output circuit of an electronic device made of, for example, ICs as shown by A in FIG. 6, the voltage at terminal 25 becomes low level when one of open-drain transistor 21 and 22 becomes turned on, and high level when both of them are turned off.
In order to attain low power consumption, the on-resistance of the pull-up circuit when one of the open-drain transistors 21 and 22 becomes turned on is designed to have a large value to suppress the drainage of current from the pull-up circuit. However, a problem arises that with a large on-resistance, a time constant becomes large and the circuit response becomes slow, the time constant being defined by the on-resistance and a capacitor indicated by a broken line and coupled to terminal 25. This problem is a burden on such a pull-up circuit that a high speed response as well as low power consumption is required. Further, as shown in FIGS. 5A and 5B, since the frequency characteristics vary with the level of Id which changes depending upon the magnitude of Vin, there arises a problem of generating waveform distortion of an input/output signal.